1. Technical Field
The invention relates to hydrocarbon recovery from a subterranean hydrocarbon-bearing formation.
1. Background Information
Kill fluids are commonly placed in a wellbore during oil field wellbore operations, including drilling, wellbore completions, and workovers, to kill the well, i.e., prevent the intrusion of formation fluids into the wellbore while the well is open. The kill fluid is often maintained in the wellbore for the entire duration of the operation.
Conventional kill fluids known in the art are typically aqueous liquids, which contain a weighting agent, such as inert inorganic solids in solution or suspension, to increase the density of the fluid. The weighted kill fluid applies a hydrostatic pressure against the formation fluid, which is greater than the pressure exerted by the formation fluid attempting to intrude into the wellbore. This overbalanced hydrostatic pressure prevents the intrusion of formation fluids into the wellbore during performance of the given oil field wellbore operation which is necessary from an operational standpoint to prevent interference from formation fluids and which is also necessary from a safety standpoint. Maintenance of a sufficient hydrostatic pressure in the wellbore reduces the potential of a well kick or blow out.
Thickeners are often included in weighted kill fluids known in the art for leakoff inhibition. See, for example, Hudson et al, SPE Paper No. 10652, which discloses a weighted brine containing a fluid loss control agent, or U.S. Pat. No. 4,391,925 to Mintz et al, which discloses a multi-phase kill fluid comprising a number of constituents including a hydrocarbon, a surfactant, a clay and an organic polymer. Advantageous kill fluids are those which prevent formation fluid intrusion into the wellbore while preventing appreciable wellbore fluid leakoff into the formation. Fluid leakoff can undesirably result in formation damage, i.e., permeability reduction, which is manifested in reduced hydrocarbon recovery from the formation or reduced injectivity into the formation.
Under downhole conditions where the wellbore is in direct communication with voids, it can be extremely difficult to prevent fluid leakoff. Conventional kill fluids generally do not exhibit sufficient flow resistance to prevent them from escaping the wellbore into the formation matrix via the high permeability voids. Such leakoff can ultimately result in loss of the hydrostatic pressure overbalance which enables formation fluids to intrude into the wellbore, negating the primary function of the kill treatment. Although it is possible to maintain the hydrostatic pressure overbalance in the face of severe fluid leakoff by replenishing the lost kill fluid, this practice can be cost prohibitive.
It is apparent that the utility of kill fluids is limited, particularly in the presence of voids in direct communication with the wellbore. Thus, a need exists for a kill treatment having utility in hydrocarbon recovery operations over a broad range of operating conditions, and particularly having utility when kill fluids are ineffective. Specifically, a need exists for a kill treatment, which is effective when severe voids in direct communication with the wellbore are present. A kill treatment is needed which prevents or minimizes the intrusion of formation fluids into the wellbore and prevents or minimizes the significant leakoff of wellbore fluid from the wellbore in the presence of such voids.